A new paper explains how atmospheric circulation will change under global warming
Key Points & Overview
- The zonal momentum budget connects tropical circulation responses to circulation responses elsewhere
- A narrowing of the ITCZ corresponds to significantly less expansion of the Hadley cell and less poleward shift of the storm tracks
- Uncertainty in the response of the Hadley cell edge and storm track position may be attributable to uncertainty in processes of the deep tropics
Understanding the response of atmospheric circulation to global warming is a central goal in climate science. In some cases, comprehensive general circulation models (GCMs) — which physically describe how the climate system operates — predict robust responses to climate change. Nearly all GCMs show a poleward expansion of the Hadley cell edge, jet stream position and storm track latitude. While these changes are a robust feature of global warming, other changes remain uncertain. For instance, it is still unknown whether the intertropical convergence zone (ITCZ) — a region where winds from both hemispheres collide and produce large amounts of precipitation — will shift northward or southward. Likewise, it is also unknown if the width of the ITCZ will change under future warming.
A new study led by Dr. Oliver Watt-Meyer, a NSERC Postdoctoral Fellow in the Department of Atmospheric Sciences, along with Prof. Dargan Frierson, aims to better understand what controls the response of atmospheric circulation to climate change. By adding slight changes to sea-surface temperatures using an idealized climate model and examining global warming scenarios, the team was able to tease out how the local atmospheric circulation response depends on nonlocal perturbations. Under global warming, the experiments in which there was narrowing of the ITCZ corresponded to significantly less expansion of the Hadley cell and somewhat less poleward shift of the storm tracks when compared to those without ITCZ narrowing. The authors’ propose that these changes can be understood through the conservation of momentum. With a narrower ITCZ, the ascending air near the equator has larger zonal momentum and so when the air moves poleward in the upper troposphere the westerly subtropical winds must get stronger. This results in a bigger difference between winds at the surface and upper atmosphere, which makes the atmosphere more unstable and lets disturbances from the midlatitudes propagate further equatorward. This mechanism means that the Hadley cell edge and position of the midlatitude jet stream and storm track will shift closer to the equator.
While this study helps uncover a new way to think about changes to atmospheric circulation, the authors’ acknowledge that their theory focuses on zonal-mean (the average across longitudes) changes of atmospheric circulation. Additionally, whether or not more fully-coupled GCMs have similar responses remains to be known. Still, this study demonstrates interactions across latitudes, which suggests that uncertainty in the response of the Hadley cell edge and storm track position may be attributable to uncertainty in process located in the deep tropics.